The concept of dual path of insertion has been applied to removable partial dentures for many years, as brought out by King, G. E., Dual-Path Design for Removable Partial Dentures, Journal of Prosthetic Dentistry, Vol. 39, No. 4, pp. 392-395 (April 1978). In the dual-path system, the first path, or approach of the metal framework of the denture to the natural teeth, is a maneuver to gain access to undercuts otherwise inaccessible from a single path of insertion. As soon as the framework has gained access to those desired undercuts, it is then rotated into a fully seated position, such rotation constituting the second and final path of insertion. Removal requires the reverse sequence of steps. Since reverse rotation of the denture must precede its removal from the undercut areas, the undercuts effectively lock the denture in place until removal is desired.
Although dentures designed to utilize the so-called "dual-path insertion" are advantageous in many respects, their popularity has been limited to some extent by the difficulty in establishing precisely those areas where tooth structure must be altered slightly to prevent interference, or where wax should be added and trimmed on a dental model (e.g., a master cast) to prevent interferences to rotational movement for the framework of the completed denture. Precision is required in ascertaining and avoiding undesired interferences, but instrumentation necessary for making such determinations has been lacking and, in general, removable partial dentures of the dual-path type have required that dental work be performed largely on a trial-and-error basis.
Instrumentation for other types of denture work are well known as revealed, for example, by U.S. Pat. Nos. 4,007,531, 2,108,980 2,528,053, 2,618,068, 2,910,773, 4,196,519, and 1,216,596. Other patents of general interest are U.S. Pat. Nos. 4,457,714, 4,474,499, 4,481,162, and 2,676,407.
An important aspect of this invention therefore lies in providing a surveying instrument, and its method of use, for establishing those relationships and modifications required for making and precisely fitting a removable partial denture utilizing the dual-path concept. The instrument includes a platform having a planar surface on which a dental model may be secured affixed. The platform includes track means which preferably takes the form of two tracks, parallel with each other and with the saggital plane of the model. A base member is adapted to be mounted for longitudinal movement upon each of the tracks and is equipped with locking means to secure it in a desired position of adjustment. The upstanding base has an upwardly-facing socket that carries a cylindrical head, the head being both rotatable and vertically extendable. A pivot shaft is mounted upon the head with its longitudinal axis extending along the plane of the support surface for the dental model, and a blade-support bar extends through a transverse opening at the tip of the bar for supporting a blade that extends along a line parallel with the longitudinal axis of the pivot shaft. Locking means are provided for securing the blade-support bar and the rotatable head in selected positions of adjustment. By aligning the tip of the shaft with the intended point of rotation (on the dental model) for insertion and removal of a partial denture, the blade-support bar may be extended and retracted to bring the blade into contact with the model's abutment teeth, and the pivot shaft may then be rotated to shift the blade along concentric arcs to establish clearances and to reveal areas where tooth material should be removed, supplemented, or trimmed, to achieve a correct and operative path of insertion and removal for a partial denture of the dual-path type.
Other features, advantages, and objects of the invention will become apparent from the specification and drawings.